The present invention generally relates to semiconductor laser control apparatuses, and more particularly to a semiconductor laser control apparatus for controlling a semiconductor laser which is used as a light source of a laser printer, an optical communication apparatus and the like.
As is well known, a semiconductor laser which is one of elements used as a light source has a driving current versus optical output characteristic shown in FIG. 1. As shown, the optical output of the semiconductor laser rises with a high fidelity to the driving current versus optical output characteristic beyond an oscillation threshold current Ith. In FIG. 1, t0 denotes an ambient temperature of the semiconductor laser, t1 denotes an ambient temperature higher than t0, L0 denotes a predetermined optical output of the semiconductor laser, Ith denotes a threshold current at a rise of the driving current versus optical output characteristic, and Ith' denotes a current at an intersection of zero optical output and an extension of the rising portion of the driving current versus optical output characteristic. In many cases, Ith' is used as the threshold current because Ith is difficult to measure in actual practice.
Hence, it is necessary to control the driving current of the semiconductor laser by use of a semiconductor laser control apparatus. A Japanese Laid-Open patent application No. 60-171863 discloses an example of the semiconductor laser control apparatus. According to this semiconductor laser control apparatus, the optical output of the semiconductor laser is detected in photodetector circuit and the detected optical output is compared with a reference value in a comparator. An up-down counter is controlled to make an up-count or a down-count depending on a result of the comparison outputted from the comparator, and the driving current of the semiconductor laser is set depending on a counted value of the up-down counter.
However, according to the above semiconductor laser control apparatus, the driving current is controlled for every predetermined time and the semiconductor laser is turned ON/OFF by an information signal thereby carrying out a two-level modulation. Since the semiconductor laser is subjected to the two-level modulation, the laser printer is limited to the printing with two gradation levels when this semiconductor laser is used as the light source and it is impossible to realize a printing with three or more gradation levels. In addition, when the semiconductor laser is used as the light source of an optical communication apparatus between a scanner and a printer and is controlled by the above semiconductor laser control apparatus, the optical communication apparatus cannot multiplex and transmit the information signal without increasing the transmission frequency.
As described above, the driving current of the semiconductor laser is set depending on the counted value of the up-down counter but the counted value of the up-down counter is outputted with reference to a timing with which the output signal of the comparator undergoes a transition from a low level or a high level. However, although the semiconductor laser control apparatus can control the optical output of the semiconductor laser to a predetermined value, the optical output is controlled with an aim merely to obtain a predetermined driving current during the operation of the semiconductor laser. In other words, when applied to the laser printer, measures are taken merely to obtain a predetermined optical output from the semiconductor laser when forming an image and it is only possible to distinguish whether or not an image exists. It is impossible to reproduce the gradation, halftone and the like of the image with a high fidelity, and there is a problem in that a reproduction with a high fidelity dependent on the state of the image cannot be achieved.
But the optical output of the semiconductor laser varies due to thermal coupling. For this reason, even when the driving current of the semiconductor laser is controlled by the above described semiconductor laser control apparatus, the optical output of the semiconductor laser once rises to a value greater than the predetermined optical output determined by the predetermined driving current when the semiconductor laser is turned ON and thereafter stabilizes to the predetermined optical output with a certain time constant. When the semiconductor laser is turned ON/OFF by a modulating signal shown in FIG. 2(A), for example, an optical output of the semiconductor laser becomes as shown in FIG. 2(C) responsive to a driving current shown in FIG. 2(B). As shown in FIG. 2(C), a change is introduced in the optical output.
When the change is introduced in the optical output of the semiconductor laser which is used as the light source of the laser printer, for example, there are problems in that inconsistencies are introduced in the gradation of the image and the halftones of the image cannot be reproduced with a high fidelity.
Recently, a digital copying machine which reads a document by charge coupled device (CCD) sensors and outputs an image data of the document to a laser printer for printing is becoming popular. The document which is copied on such a digital copying machine includes photographs and pictures having gradation. And when outputting to the laser printer the image data including gradation information, the known dither technique using the dither matrix is used to describe the gradation levels. In other words, one picture element is constituted by a 4.times.4 matrix and each picture element is turned ON/OFF (two-level) so as to describe the gradation in 16 (=4.times.4) gradation levels.
In a case where it is necessary to describe the gradation in a large number of gradation levels, the size of the dither matrix is increased. For example, a 8.times.8 dither matrix is used to describe the gradation in 64 gradation levels. However, according to this method, the picture elements become coarse because of the need to increase the size of the dither matrix when the gradation levels of the gradation which is to be described increases and there is a problem in that the resolution becomes poor.
FIG. 3 shows an example of the conventional semiconductor laser control apparatus, and FIGS. 4(A) through 4(C) are timing charts for explaining the operation of this conventional semiconductor laser control apparatus.
In FIG. 3, an image signal (or data) is applied to an input terminal 11 and supplied to a base of a transistor TR1 through an inverter 12. The image signal is also supplied to a base of a transistor TR2 through a non-inverter 13. A collector of a transistor TR3 is connected to emitters of the transistors TR1 and TR2, and an emitter of the transistor TR3 is grounded through a resistor R1. A voltage V.sub.RF is applied to a terminal 14 and supplied to a base of the transistor TR3. The transistor TR3 and the resistor R1 constitute a constant current supply circuit.
A collector of the transistor TR1 is connected to a power source for supplying a power source voltage Vcc, and a collector of the transistor TR2 is coupled to the power source through a semiconductor laser LD. The collector of the transistor TR2 is also coupled to a collector of a transistor TR4 through a coil L. An emitter of the transistor TR4 is grounded through a resistor R2, and a voltage V.sub.B is applied to a terminal 15 and supplied to a base of the transistor TR4.
When an image signal shown in FIG. 4(A) is applied to the input terminal 11, the transistors TR1 and TR2 are turned ON alternately. That is, the transistor TR1 is turned ON when the transistor TR2 is turned OFF and the transistor TR1 is turned OFF when the transistor TR2 is turned ON, and such turning ON/OFF of the transistors TR1 and TR2 is repeated. When the transistor TR2 is ON, a current I.sub.RF from the constant current supply circuit made up of the transistor TR3 and the resistor R1 and a bias current I.sub.B due to the transistor TR4 and the resistor R2 flow to the semiconductor laser LD. When the transistor TR2 is OFF, the bias current I.sub.B flows to the semiconductor laser LD. Accordingly, a pulse current shown in FIG. 4(B) flows through the semiconductor laser LD and the optical output of the semiconductor laser LD takes a pulse form as shown in FIG. 4(C). In FIGS. 4(B) and 4(C), phantom lines respectively denote a zero current and a zero output.
Hence, the semiconductor laser LD is modulated depending on a pulse width of the image signal applied to the input terminal 11. In order to obtain an image having the gradation described in a large number of gradation levels without sacrificing the resolution, it is possible to give two or more levels with respect to one dot by varying the modulation pulse width of the semiconductor laser LD. But such a description of the gradation is still insufficient.